The sounds of high winds

For = 0 equation (III.2) reduces to V h,log = (u * /)·ln(h/z o ), the widely
used logarithmic wind profile. With this profile the ratio of wind velocities
at two heights can be written as:
V h2,log /V h1 = log(h 2 /z o )/log(h 1 /z o )
(III.3)
For a roughness length of z o = 2 cm (pasture) and m = 0,14, the wind
profiles according to equations III.1 and III.3 coincide within 2% for h <
100 m. In figure III.1
wind profiles are given
as measured by
Holtslag [1984], as well
as wind profiles
according to formulae
(III.1) and (III.3).
Formula III.3 is an
approximation of the
wind profile in the
turbulent boundary
layer of a neutral
atmosphere, when the
air is mixed by
turbulence resulting
height (m)
100
80
60
40
20
0
logarithmic profile:
atmosphere
unstable / neutral
0 2 4 6 8 10
wind speed (m/s)
from friction with the surface of the earth. In daytime thermal turbulence is
added, especially when there is strong insolation. At night time a neutral
atmosphere, characterized by the adiabatic temperature gradient of -1 ºC
per 100 m, occurs under heavy clouding and/or at relatively high wind
velocities. When there is some clear sky and in the absence of strong winds
the atmosphere becomes stable because of radiative cooling of the surface:
the wind profile changes and can no longer be adequately described by
(III.3). The effect of the change to a stable atmosphere is that, relative to a
given wind velocity at 10 m height in daytime, at night there is a higher
wind velocity at hub height and thus a higher turbine sound power level;
also there is a lower wind velocity below 10 m and thus less wind-induced
sound in vegetation.
30
atmosphere
stable
Figure III.1: wind profiles